Measuring who our carbon footprint landing on

Citation: Edmonds, D.A., Caldwell, R.L., Brondizio, E.S. et al. Coastal flooding will disproportionately impact people on river deltas. Nat Commun 11, 4741 (2020). https://doi.org/10.1038/s41467-020-18531-4

How do you measure and communicate the impact of a changing climate? One way is through meteorological measurements and graphs. You can present historical data of rising CO₂ levels, and point towards seemingly irrefutable data that cautions of more frequent tropical storms and a future of degrading coasts. A different approach is one of the studies of human impacts as they answer the question: How is climate change affecting real people now and in the future?

Life in a river delta pounded by climate change

Millions of people live in vulnerable areas known as river deltas. Because of this, millions of people are expected to be impacted by changes in sea level. Using a new global dataset of river delta area based on high resolution images from Google Earth, researchers at Indiana University suggest that 339 million people lived on river deltas in 2017 and 89% of those people live in the same latitudinal zone as most tropical cyclone activity. A disproportionate amount of 329 million of those in these deltas live in developing or least developed economies that would not have the resources to properly withstand or recover from devastation.

This study set out to highlight scientific data proving how some of the poorest regions of the world receive the greatest burden of climate disturbances, such as increased storms and flooding. Framing climate change in this way, bringing attention to the special physical and socioeconomic challenges these areas face, may help organizations take more focused and appropriate actions against flood mitigation.

Researchers have had difficulty defining an area that is constantly changing

It’s difficult to calculate the human impact of climate change in locations where the population is not properly identified. River deltas, as important as they are as locations of visible climate change, suffer from inadequate and inconsistent definitions. Very broadly, a river delta forms when rivers carry sediment into stagnant water where it cannot move away. They are often changing as sediment enters and leaves. Without a clear definition of where these vulnerable areas exist, it is impossible to put quantitative numbers to the amount of people being affected. To address this challenge, Sacha Siani and colleagues developed a new global dataset of delta area to define the global deltaic population, and its vulnerability to flood hazards.

Deltas are naturally near sea level and their combination of low elevation and dense infrastructure degrades the land even more. Stress is further applied by the fact that many delta river communities experience severe economic stressors associated with limited resources, poverty, and inadequate public services which hamper any flood mitigation efforts.

However, thousands of years of land use have caused natural land degradation like erosion and infrastructure damage such as inadequate storm drainage. Coupled with changing weather events and rising sea levels, the future may no longer be as bright for these once vibrant communities.

Researchers use Google Earth to define river deltas

First, Sacha Siani and colleagues defined a “delta” by assigning consistent delta boundaries along relevant costs of every continent except Antarctica using Google Earth images. The definition and subsequent area calculations were based on 5 points. If each of these 5 areas were observed, the area encompassed in those points was considered a delta:

1. River mouth (RM): The widest river mouth on the shoreline
2. Delta node (DN): The most upstream branch in the water channel or the intersection of the main channel with the shoreline vector [The line connecting S1 and S2 as defined in (3) and (4)]
3. Lateral shoreline extent point (S1): Point on shoreline that marks boundary between delta and non-delta shoreline or lateral most extent of water channel activating (left side looking upstream)
4. Lateral shoreline extent point (S2): Point on shoreline that marks boundary between delta and non-delta shoreline or lateral most extent of water channel activating (right side looking upstream)
5. Basinward extent point, toward open basin (OB): Delta land located furthest into the main water basin, perpendicular to the shoreline vector [The line connecting S1 and S2 as defined in (3) and (4)]

This definition allowed them to make broad statements about population and climate change impacts on the global scale. They calculated that although deltas occupy only 0.57% of the Earth’s land surface area, they contain 4.5% of the 2017 global population. To provide an idea of just how densely populated these areas are, the researchers hypothetically distributed all 339 million people living in river deltas evenly across all habitable delta regions and the population was still 8 times the global average.

Who is in danger?

Cross-referencing with historical information about flooding, the researchers suggest that both physical and socioeconomic factors must be responsible for determining a population’s vulnerability to climate change.

Physically, the majority of residents in river deltas are prone to storm surges, while 9.1% live in 100-year storm surge floodplains. Contrary to popular thought that 100-year floods only happen once every 100 years, the actual definition is that these events have a 1% chance of occurring each year. Socioeconomically, 97% of people living on deltas are doing so in developing or least-developed economies and are more vulnerable to coastal flooding. High poverty rates, poor infrastructure, and lower quality of life severely limit the extent to which communities can realistically manage flood risk. Most of these river deltas are in Asia-Pacific region, in areas like Ganges–Brahmaputra and the Mekong. 

Coastal flooding is a problem almost everywhere. Some river delta communities, like the Mississippi and the Nile, have engineered temporary solutions like levees or dams. The researchers note that these synthesized designs have use, but are also susceptible to failing. A natural solution would be to allow the river room to deposit additional sediment in areas where the land is failing. However, some of the most sediment-stressed floodplains are so large that natural solutions are no longer feasible.

The researchers suggest that the science community should frame coastal flooding as a problem that disproportionately impacts people on river deltas in developing and least-developed economies. The goal would be to bring attention to the special physical and socioeconomic challenges these areas face in terms of flood mitigation. If anything, they believe their calculations are too low in terms of how many people in these areas are affected by coastal flooding. They recommend scientists develop better elevation and storm surge models as well as simulated flooding events in densely packed deltas to more accurately assess the risks to these vulnerable populations.

Framing vulnerable populations through both a geographical and socioeconomic lens

We can talk about climate change in terms of data and graphs, but many people in developed countries have never experienced devastating changes in their climate change first hand. In part, this is because the most damage is currently occurring to the most vulnerable, in regions far removed from the popular media’s eye. Perhaps it is not the spewing of more information that the world needs to see to spur changes in the way developing countries prioritize energy. It may be studies like the one here, that combine science and public health, that will provide the most insightful glimpses into human impact. At the end of the day, the numbers will only be what helps scientists and engineers mitigate the damage. The human impact will be what helps people listen. 

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